You don,t need to run anyhing else for auth. This is done in the custom oAuth2 and Bubble will automatically add the authorization header to all call. You can delete the token endpoint but set this in oAuth2 custom token call like in my screenshot.
Simple get user call to validate my auth is working
If you look correctly, you need to call the correct server endpoint for this type of auth. It’s not the same as user-agent/authorization_code flow (UPDATE: tested with https://login.salesforce.com/services/oauth2/token and it’s also work fine)
And best is to use client_credentials if possible. This is also done in custom auth if I remember